1. Field of the Present Invention
The present invention relates to a light spot position control apparatus for controlling spot positions of light irradiating a disk-shaped recording medium, and more particularly, to a light spot position control apparatus which is suitable for controlling information recording/reproducing positions based on irradiation of light different from information recording/reproducing light and a method thereof.
2. Description of the Related Art
For example, Japanese Unexamined Patent Application Publication Nos. 2005-250038 and 2007-79438 disclose a hologram recording/reproducing system which performs data recording by using an interference fringe between a signal light and a reference light. In the hologram recording/reproducing system, at the recording time, the interference fringe is formed on a hologram recording medium by allowing the signal light, which is subjected to spatial light modulation (for example, light intensity modulation) according to recording data, and the reference light, which is different from the signal light, to irradiate the hologram recording medium, so that the data recording is performed.
In addition, at the reproducing time, the hologram recording medium is irradiated with the reference light. In this manner, by irradiating the hologram recording medium with the reference light, a refracted light corresponding to the interference fringe formed on the hologram recording medium can be obtained. In other words, therefore, a reproducing light (reproducing signal light) corresponding to the recording data can be obtained. By detecting the obtained reproducing light with, for example, an image sensor such as a CCD (charge coupled device) sensor or a CMOS (complementary metal oxide semiconductor) sensor, the recording data are reproduced.
Similarly to a recording/reproducing system for an optical disk such as a CD (Compact Disc) or DVD (Digital Versatile Disc) in the related art, in a hologram recording/reproducing system, data are considered to be recorded in tracks formed on a recording medium. In other words, similarly to the case of the optical disk in the related art, recording/reproducing position control such as tracking servo control is performed on the tracks, so that the data are recorded at appropriate positions on the disk.
An example of a structure of the hologram recording medium used in the case of performing the recording/reproducing position control is described with reference to a cross-sectional structural view of FIG. 17.
FIG. 17 illustrates an example of a structure of a reflective hologram recording medium 100 having a reflecting layer.
As shown in the figure, in the hologram recording medium 100, a recording layer 106 where a hologram is recorded by the aforementioned interference fringe between the signal light and the reference light and a position control information recording layer where address information or the like for position control by a cross-sectional structure of convex-concave portions on a substrate 110 is recorded are separately formed.
More specifically, in the hologram recording medium 100, a cover layer 105, the recording layer 106, a reflecting layer 107, an intermediate layer 108, a reflecting layer 109, and the substrate 110 are formed in this order from the upper layer.
When a reproduced image corresponding to the hologram recorded on the recording layer 106 is obtained by irradiating a laser light (the aforementioned reference light) for hologram reproducing at the reproducing time, the reflecting layer 107 formed as the lower layer of the recording layer 106 is used to return the reproduced image as a reflected light to the apparatus side.
In addition, tracks for guiding the hologram recording/reproducing positions of the recording layer 106 are formed on the substrate 110 in a spiral or concentric shape. For example, the tracks are formed by recording information such as address information by using pit columns.
The reflecting layer 109 formed as the upper layer of the substrate 110 is provided so as to obtain a reflected light corresponding to the pit columns. In addition, the intermediate layer 108 is made of, for example, an adhesive material such as a resin.
The recording/reproducing light for performing the hologram recording/reproducing on the recording layer 106 and the position control light for obtaining the reflected light from the position control information recording layer are allowed to individually irradiate the hologram recording medium 100 having the cross-sectional structure.
Herein, if only one light is used simultaneously for the hologram recording/reproducing and the position control, at the reproducing time, the component corresponding to the cross-sectional shape of convex-concave portions of the substrate 110 (reflecting layer 109) is overlapped as a noise with the reproduced image of the hologram, so that the reproducing performance may deteriorate. For this reason, in the hologram recording/reproducing system, the position control is performed so that the position control light for obtaining the reflected light from the position control information recording layer together with the hologram recording/reproducing light is allowed to individually irradiate.
In addition, in the case where the position control light is irradiated separately with the hologram recording/reproducing light, the different wavelength bands thereof are used. If the position control light and the recording/reproducing light in the same wavelength band are used, the photo-sensing of the recording layer 106 may occur due to the irradiation of the position control light. In order to prevent this phenomenon, the different wavelength bands are used.
For example, a blue-violet laser light having a wavelength λ of about 405 nm is used as the hologram recording/reproducing light, and a red laser light having a wavelength λ of about 650 nm is used as the position control light.
Herein, in order to obtain the reflected light from the position control information recording layer through the irradiation of the position control light, the position control light necessarily reaches the reflecting layer 109, which is formed by reflecting the cross-sectional shape of convex-concave portions of the substrate 110. In other words, the position control light necessarily transmits the reflecting layer 107, which is formed above the reflecting layer 109.
On the other hand, the reflecting layer 107 necessarily reflects the hologram recording/reproducing light so that the reproduced image corresponding to the hologram recorded on the recording layer 106 returns to the apparatus side as a reflected light.
By taking this point into consideration, a reflecting layer having a wavelength selectivity, which reflects the blue-violet laser light as the aforementioned recording/reproducing light and transmits the red laser light as the aforementioned position control light is used as the reflecting layer 107. Accordingly, the position control light reaches the reflecting layer 109, so that the reflected light for the position control appropriately returns to the apparatus side; and the reproduced image of the hologram recorded on the recording layer 106 is reflected on the reflecting layer 107, so that the reflected light appropriately returns to the apparatus side.
Here, in the case where the recording/reproducing position control is performed by using the light separately from the hologram recording/reproducing light, as shown in FIG. 18, in the recording/reproducing apparatus side, the hologram recording/reproducing light and the position control light are combined in the same optical axis, and the combined light is allowed to irradiate the hologram recording medium 100. In addition, the tracking servo control is performed based on the reflected light of the position control light.
In this manner, the hologram recording/reproducing light and the position control light are combined on the same optical axis, and the combined light is allowed to irradiate the hologram recording medium 100. Next, the position control is performed based on the reflected light of the position control light, so that the hologram recording/reproducing position is controlled to be the position along the tracks (pit column) formed on the hologram recording medium 100.
However, the aforementioned method of controlling the hologram recording/reproducing position in the related art is a method based on the assumption that the optical axis of the position control light and the optical axis of the recording/reproducing light are coincident with each other. Therefore, for example, in the case where there occurs a misalignment between the two optical axes due to an aging change or a temperature change as shown in FIG. 19, the hologram recording/reproducing position may not be controlled to the accurate position according to the track.
In other words, as understood from this point, in the hologram recording/reproducing system of the related art, which performs the position control by using the aforementioned method, at the reproducing time, the recoded hologram column may not be accurately traced. As a result, the hologram reproducing may not be appropriately performed.
In addition, the problem of the misalignment between the hologram recording position and the reproducing position caused by the misalignment between the two optical axes may occur even in the case where the reproducing is performed on the disk on which the recording is performed by the different apparatus other than the current apparatus. For example, in the case where the axial misalignment amount between the position control light and the recording/reproducing light at the hologram recording time in the different apparatus is α, if the axial misalignment amount between the position control light and the recording/reproducing light in the current apparatus is β, the hologram recorded by the different apparatus may not be appropriately reproduced by the current apparatus.
Therefore, a procedure for correcting the misalignment between the hologram recording position and reproducing position is considered to be employed.
As a detailed procedure, there is a procedure of separately providing an adjusting mechanism such as an actuator for adjusting the axial position of the recording/reproducing light and, at the reproducing time, adjusting the axial position of the recording/reproducing light (reference light) so as to be coincident with the actual position at which the hologram is recorded.
More specifically, in the case where this procedure is employed, before the hologram reproducing is performed, calibration for specifying the axial position at which the largest reproducing light amount is to be obtained is performed by shaking the axial position of the recording/reproducing light (reference light) by the actuator (adjusting mechanism). In other words, accordingly, the position at which the hologram is actually recorded may be specified. Next, by adjusting the axial position of the recording/reproducing light to the obtained position, the misalignment between the hologram recording position and reproducing position may be corrected.
However, in the case where the procedure is employed, the actuator (adjusting mechanism) for adjusting the axial position of the recording/reproducing light is necessarily provided. Therefore, the production cost of the apparatus is increased.
In addition, a very high accuracy of adjustment is necessary to the correction of the misalignment between the recording position and the reproducing position. At the hologram reproducing time, although the misalignment in the position of irradiation of the recording/reproducing light (reference light) on the recorded hologram is small, the deterioration in the refraction efficiency (namely, the deterioration in the reproducing light amount) may occur. More specifically, a very high accuracy of adjustment, for example, an accuracy of about a submicron is necessary for the correction of the axial position of the recording/reproducing light.
Since such a very high accuracy of adjustment is necessary, in the case where the procedure of providing a separate adjusting mechanism for adjusting the axial position is employed, the technology thereof may be very difficult to implement. In addition, the adjusting mechanism may be a highly-accurate, highly-strong mechanism, so that the production cost of the apparatus is further increased.